1. Field of the Invention
The present invention relates to turbo-machines, more particularly means for securing and for limiting the angular clearance of the rotor blades of fans and compressors.
2. Description of the Prior Art
Turbo-machine rotor blades are secured in a known mannner on the periphery of a rotor disc and conventionally include, beneath a platform and separated therefrom by a stilt, a root of a fir-tree or dovetail form engaged in a correspondingly shaped groove of the rotor disc. For assembly reasons, when blades with fins or peripheral platforms are concerned, the designer is required to provide a substantial space between the blade root and the bottom of the groove. Such space is occupied by an axial wedge which provides correct location of the root of each blade in the corresponding grooves.
An assembly of this kind is illustrated for example in French patent specification No. 2 345 605 in the name of Societe Nationale D'Etude et de Construction de Moteurs.
Such blades are thus liable when at rest to have a relatively substantial angular clearance on one side and on the other of a radial plane including the major axis of the blade, such clearance being kept at a given value by the calculated spacing between the platforms of adjacent blades.
However, problems arise for fan or low pressure compressor blades of a turbo-machine, particularly an aircraft jet engine because the fan rotor or low pressure compressor rotor lies at the upstream end of the turbojet engine, and may be subjected to shocks in flight resultant from the ingestion of birds and under the tangential force of the shock of the blade, the latter being subjected to deflection and simultaneously subjected to rotation about its major axis because of the substantial degree of freedom in rotation permitted at the root of the blade as has been discussed hereinbefore.
If the mass of the ingested bird is substantial, the energy of the shock can be sufficient to lead to partial break up of one or more blades, or even their complete fracture, which may lead furthermore to destruction of downstream stages of the compressor or fan.
This disadvantage is further accentuated for fan blades having fins at mid-height of the blade, because under the effect of the tangential force resulting from the shock, the adjacent fins override one another, which will lead with near certainty in the destruction of the fan blades and as a result the partial or total destruction of the downstream stages of the compressor, which in turn may lead to the complete stopage of the turbojet engine and in the worst cases, particularly if the aircraft has only a single engine, leading to the loss of the aircraft.
An example of this kind of assembly is disclosed in French Pat. No. 2 286 282 which features rotor blades of which the platforms are engaged upon the edges of a cage, the latter being based upon two discs, one upstream, the other downstream interconnected by narrow rails or bars extending axially and disposed beneath the platforms in a space between two adjacent platforms which remains free between the stilts, the root of the blade being engaged in a groove of the rotor disc and the cage locked on the rotor disc.
Such device where the blade root is not wedged in its groove gives rise to the disadvantage of permitting substantial rotation of the blade in the groove of the disc and of leading to substantial vibration between the blades and the cage on which the platforms rest.
Attempts have therefore been made to suppress such vibrations especially by providing, beneath the platforms of the blades in the space remaining free between the stilts, wedges engaged beneath the platforms, either by centrifugal force during rotation of the rotor, or as is taught by British Pat. No. 670 665 or U.S. Pat. Nos. 4,182,598 or 4,101,245, or by a resilient means as disclosed in French Pat. No. 2 527 260 in the name of Societe Nationale D'Etude et de Construction de Moteurs.
However, even if such devices fulfil an anti-vibration function by limiting the radial displacement of the blades, in general they do not limit the angular twisting of the latter under the shock action. Furthermore such devices cannot absorb the energy of the shock on a blade in order to transmit it and to distribute it for example over a whole sector of the disc where the energy would be dissipated.